Epidermal growth factor (EGF) stimulates cell division in many cell types including some differentiated teratocarcinoma cells but in no case has a stem cell (embryonal carcinoma, EC cell) been found to respond. Only cells which have the highly specific EGF receptor can respond. The characteristics of binding and of the receptor molecule will be studied in teratocarcinoma cells and these will be compared with embryo-derived cell lines. The mechanism of the signal that EGF imparts to the cell will be probed using liposomes and membrane extracts. The responses of cells will be measured in terms of increased growth rate, DNA synthesis or differentiation (expression of alphafetoprotein, transferrin, collagen type, basement membrane proteins). It is intended to identify the cells within an embryo which can bind and which may respond to EGF either in vitro or in vivo. The aim is to study the role of EGF and its receptors during development of the mouse embryo. A postulated EGF-like factor will be sought and assayed using both radioimmunoassay and radioreceptorassay.